Recently, hollow inorganic micro- and nanostructures have attracted considerable attention because of their promising applications as nanoscale chemical reactors, catalysts, drug delivery earners, semiconductors, and photonic building blocks. X. W, Lou, C. Yuan, Q. Zhang, L. A. Archer, Angew. Chem, Int. Ed. 2006, 45, 3825. In particular, nanostructures comprising metal oxides may be used as semiconductors in applications such as gas sensors and lithium rechargeable batteries. Y. Wang, X. Jiang, Y, Xia, J. Am. Chem, Soc. 2003, 725, 16176; M. Law, H, Kind, B. Messer, P. Kim, F, Yang, Angew. Chem. Int. Ed. 2002, 41, 2405; Y. Idota, T, Kubota, A, Matsufuji, Y. Maekawa, T. Miyasaka, Science, 1997, 276, 1395. However, although metal oxides (for example, tin oxide (SnO2)) generally have a much higher theoretical specific lithium storage capacity area than more traditionally used materials such as graphite, the large volume changes in metal oxide nanostructures during charging/discharging processes result m poor cyclability, thus limiting their use in such applications. S. Han, B. Jang,. T. Kim, S. M. Oh, T. Hyeon, Adv. Funct. Mater, 2005, 15, 1845; K. T. Lee, Y. S. Jung, S. M. Oh, J. Am. Chem. Sac. 2003, 125, 5652; Y. Wang, H. C. Zeng, J. Y. Lee. Adv. Mater, 2006, 18, 645, One possible strategy to mitigate this problem and further enhance structural stability is to use hollow nanostructures. Such hollow nanostructures have higher surface to volume ratios which allow for greater charge capacities than solid nanostructures. Hollow nanostructures may be designed in various shapes and sizes, and commonly include nanoparticles, nanorods/belts/arrays, nanotubes, nanocylinders, nanococoons, nanodisks, nanoboxes, nanospheres, among others.
At present, there exist numerous methods for the preparation of hollow metal oxide nanostructures. One approach involves the use of various removable or sacrificial templates, including, for example, monodispersed silica, carbon or polymer latex spheres, reducing metal nanoparticles, and even emulsion droplets/micelles and gas bubbles. Q. Peng, Y. Dong, Y. Li, Angew. Chem. Inl. Ed. 2003, 42, 3027-3030. Although conceptually simple and versatile, such methods are often burdened with the challenge of uniformly depositing metal oxides (or their precursors) on templates, a problem which has traditionally been dealt by prior surface modification, itself a tedious process, M. Yang, J. Ma, C. Zhang, Z. Yang, Y. Lu, Angew. Chem. Int. Ed. 2005, 44, 6727. Other templating methods such as templating sol-gel precursors with colloidal crystals or their replicas (often called the “lost-wax approach”) result in amorphous and structurally fragile nanostructures upon crystallization at high temperatures and upon template removal. Z, Zhong, Y. Yin, B. Gates. Y. Xia, Adv. Mater, 2000, 12, 206-209.
Other approaches, which are template free, employ mechanisms such as corrosion-based inside out evacuation and the nanoparticle Kirkendalf effect. Y. D. Yin, R. M. Rioux, C. K, Erdonmez, S. Hughes, G. A. Somorjai, A. P. Alivisatos, Science 2004, 304, 711. However, many such existing methods for the production of nanostructures are often cumbersome involving multiple steps that are often difficult to control, and are cost-prohibitive which prevent them from being used in large-scale applications. Moreover, many of these existing methods also result in poor yields of mono-disperse, hollow nanostructures, producing mixed hollow and solid nanostructures, or nanostructures with large-size distributions.
Thus, there exists a need for improved hollow metal oxide nanostructures comprising a relatively high surface to volume ratio (and thus a large number of potential active sites) and physical stability. There also exists a need for viable industrially sealeable methods of producing such hollow metal oxide nanostructures at low cost, high yields, narrow-size dispersions, geometric stability and homogeneous morphologies.